In the acetylaminofluorene/partial hepatectomy (AAF/PH) model both stem cell factor (SCF) and its receptor c-kit were present in the oval cells. There was a correlation between the level of SCF and the extent of oval cell proliferation, while neither simple PH nor AAF administration alone induced a noticeable expression of the SCF/c-kit. Separation of nonparenchymal cells from parenchymal cells at day 3 in the AAF/PH model demonstrated that the expression of both SCF and c-kit receptor genes was restricted to the nonparenchymal cells. Both cellular proliferation and programmed cell death (apoptosis) were simultaneously present in hepatocytes of vitamin A deficient livers. Apoptosis was greatly enhanced in hepatocytes a few hours after PH of vitamin A deficient animals, whereas no apoptotic cell death was observed in the bile duct cells. Prolonged and increased levels of expression of the c-jun, c-fos, and c-myc genes were also observed. Fibroblast growth factor receptor gene, flg, is absent in the adult hepatocytes but is present in the nonparenchymal fraction, whereas another receptor, bek, is present in both fractions. After partial hepatectomy, only the transcripts for bek showed a significant increase. As compared to the adult liver, flg was expressed at a high level in embryonic livers, whereas a significant increase of bek expression occurred at the postnatal stage of liver development. In the AAF/PH model, both bek and flg were expressed at high levels, flg being mainly localized in the oval cells, whereas bek was expressed in both oval cells and Ito cells. The transcripts for both retinoic acid receptor(RAR)-alpha and RARbeta increased significantly when the proliferation and differentiation of oval cells were most prominent. The transcripts for both receptors were present mainly in oval cells and Ito cells. However, RARalpha was most prominent in the oval cells and in the OV-6 positive cells in hepatocellular carcinoma.